In the pressure gauge arts, pressure gauges employing precision piezoelectric quartz resonators are known for providing highly accurate pressure determinations. U.S. Pat. No. 3,561,832 issued to Kerrer et al., and assigned to Hewlett-Packard Company, shows a quartz pressure gauge which is created from a single piece of crystalline quartz with the gauge comprising a hollowed cylinder and a circular resonator with electronics thereupon. The cylinder may be oriented with respect to the crystalline access for selected characteristics. By applying a predetermined voltage to the resonator, the resonator may be caused to resonate at a resonance frequency. The application of pressure around the cylinder causes the resonator to change its oscillation frequency, thus by measuring the change in oscillation frequency, the applied pressure may be accurately determined.
While the oscillation frequency of the resonator of the quartz pressure gauge is a function of the applied pressure, it is known that the frequency is also affected by temperature. Thus, in order to provide an exact measure of the applied pressure, the reading must be compensated for temperature changes. One prior art method for eliminating the temperature effect on the pressure measuring gauge is to provide an isolated second reference gauge which is kept at a constant pressure but which is subjected to the same temperature fluctuations as the first gauge. When both gauges have attained thermal equilibrium at the same temperature, the difference in frequencies output by the gauges is directly correlated to the pressure measured by the first gauge.
The above described technique is effective only after both gauges have reached the same temperature equilibrium. It is ineffective when temperature transients occur, since the second gauge is likely to reach equilibrium at a slower rate than the first gauge. This is because the second gauge must be surrounded by some material which protects it from the pressure fluctuations, and this material will also provide some thermal isolation. In addition to thermal transients occurring in the fluid or gas being measured, if the fluid or gas changes pressure, this pressure change may induce an adiabatic temperature change in the gauge that is subject to pressure. The second gauge, however, which is isolated from the pressure change, would not undergo the adiabatic change.
U.S. Pat. No. 4,607,530 issued Aug. 26, 1986 to Chow, entitled "Temperature Compensation for Pressure Gauges" discloses one prior art method for compensating for thermal transients. This method relies on the physical construction of the gauge, and upon creating a dynamic model that compensates for the flow of heat through the physical exterior of the gauge to the interior where the pressure is being measured. The method uses a fast responding thermal device on the exterior of the gauge to measure the temperature at the exterior, and uses the model to predict the temperature on the interior of the gauge at the location of the resonator. By calibrating the gauge to provide a known relationship between the frequency of the resonator and the temperature of the resonator, the system then uses the dynamic model to predict the temperature of the resonator based upon the temperature of the fast acting thermal device. Thus, as the temperature around the gauge changes, the model, in combination with the fast responding thermal device, will predict the temperature of the resonator. The resonator temperature can then be used to determine the pressure error due to the temperature.
The method of Chow suffers from the limitation that the dynamic model is based on the construction of the particular gauge being used to perform the measurement. Thus, this method may not easily be adapted to other manufacturer's gauges, or to other types of gauges.
"State-of-the-Art Simultaneous Downhole Flow Rate and Pressure Measurement Equipment" by G. W. Haws and B. L. Knight, published at the 65th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Sep. 23, 1990, describes thermally compensating pressure gauges using a differential equation which is a modification of an RC electrical circuit description. "Pressure Gauge specification considerations in Practical Well Testing", by A. F. Veneruso, et al., published at the 66th Annual Technical Conference and Exhibition of the Society of Petroleum Engineers, Oct. 6, 1991, also describes thermal compensation of pressure gauges using a differential equation similar to the Haws and Knight equation. Both these methods are limited in their accuracy.
It is thus apparent that there is a need in the art for an improved method of compensating for transient temperature changes while measuring pressure in an oil or gas well. There is further need in the art for such a method that does not depend upon the physical construction of the gauge used during the measurement. The present invention meets these and other needs.